In Apps Script, a deployment specifies a snapshot of the project code that an add-on, web app, or executable uses. Deployments let you to manage what version of your code is available for others to use while allowing you develop on a newer version.
For example, you can publish a Gmail add-on version to the G Suite Marketplace where others can find and install it. You can develop the next version of the add-on in the same script project without interfering with those using the original version. When the next version is ready, you can update the deployment to use that version. Once the deployment is updated, the new version is used when users next open your add-on (no re-publish or re-install required).
Find more details about creating and managing deployments in the Apps Script Deployments guide.